The increasing parallel use of ICD-9 and ICD-10 complicates the comparability of coded diagnoses. This is the reason why we developed a symmetric table for interactive conversion between ICD-9 and ICD-10, based on a vector space text-retrieval method that resulted in unambiguous mapping from ICD-9 to ICD-10 in 64%, from ICD-10 to ICD-9 in 87% of all three- and four-character classes of the tabular list. Out of the remaining 13% of multi-valued relations, a table for automated mapping from ICD-10 to ICD-9 was created. In 9% of cases, the selection offered no problems. A compromise between preserving information content and maintaining the logical integrity had to be found in 2.4%; in 1.6% automated mapping was impossible because of newly defined concepts and structural differences between ICD-9 and ICD-10 that are not counterbalanced by a consistent system of residual categories. We recommend that in a future revision of the ICD, compatibility with the then existing classification system should be considered.